Industrial Wheel Design OptimizationIndustrial Wheel Design Optimization
September 30, 2010
Despitethe established physical and dynamic properties of specific materials (rubber,nylon and polyurethane), there are a number of design application issues toconsider in generating optimum wheel performance for demanding applications. Manychanges can be made to improve performance and extend the usable life ofindustrial wheels that are experiencing frequent failures (such as dynamicfailures known as blowouts, delaminations or fatigue life). Some of thesechanges include the use of alternative polyurethane chemistries, wheel designchanges or even altering manufacturing parameters.
Thereare many aspects of an industrial application to consider to determine theappropriate wheel design, material and manufacturing parameters to optimizewheel performance. Some of these considerations include:
General operating conditions of the application/system and the running surface condition (including joints, paint, lubricants, proper alignment, etc.);
Load and speed profile of the application including the duration of cycles and idle time between cycles;
Wheel type (i.e., drive/brake wheels, guide wheels or free running load wheels);
Dimensional constraints of the application;
Vibrations or abrasions that occur during use;
Environmental conditions including exposure to water, chemicals or other contaminants, ambient weather conditions and ultraviolet light exposure; and
Application serviceability and the wheel's targeted life span and preferred mode of failure. For existing applications, evaluating the design and performance of existing wheels and common modes of failure is critically important--how a wheel fails can be as important as when it fails.
Thefollowing case study describes a solution provided for a major retailer that improvedthe operating performance of the drive wheels for a large automated materialshandling crane located in the company's largest warehouse/distribution facilityin the U.S. In this case, the existingsolid wheels were constructed from another polyurethane material.
OnsiteEvaluation and Testing
Followingthe initial project review with engineers at the distribution facility, Uremet (asupplier of high-performance polyurethane wheels) conducted an onsiteevaluation of the automated materials handling crane system and its drivewheels. In addition to generalobservations relating to the operating environment and conditions, multipletests were conducted and data gathered during the initial evaluation, includingmeasuring the operating temperature of the existing wheel treads and theambient temperature of the facility. Details regarding the load, acceleration,maximum speed, operating cycle, crane weight and wear requirements were noted.The existing wheels were examined for wear, fatigue and delamination.
Ageneral review of the historical operating performance, life span and observedfailure modes for the existing wheel tread was also discussed with the facility'sengineers and maintenance technicians. The average tread life of the existingwheels was estimated to be less than three months, which caused significantdowntime issues for the system. The existing wheel treads typically failed dueto either a form of delamination known as bondline fatigue or severe fatiguecracking of the tread material. Bondline fatigue is a failure generally causedby overloading, fatigue or overheating of the bondline resulting in areas ofseparation between the hub and the tread. Fatigue cracking is a failuretypically caused by high-stress concentrations during cyclic loading. In bothinstances, the vibration induced by these failures was believed to be causingfatigue to the crane frame structure and premature failure of the drivetraingearboxes. In addition, the existing wheels were only available as new completewheel assemblies sourced from Europe, resulting in long lead times and highreplacement costs.
Additionalanalysis was performed including the examination of the customer supplied wheelsamples. The wheel tread material was confirmed to be an NDI/polyester-basedpolyurethane with a hardness of 96 Shore A. Using the data gathered during the onsiteevaluation, the application was analyzed using computer models for targeteddeflections, traction, bondline stresses and load capacity based on historicalexperience and previous testing results from Uremet's application simulationtester. It was determined that the configuration of the existing wheel designwas satisfactory and that an adjustment to the formulation of the polyurethanetread would provide enhanced performance.
Uremet'sU3000 polyurethane, with a hardness of 78 Shore B, was selected as the optimumtread for this application. Compared to the original material, the U3000material has a lower compressive modulus, lower hysteresis, higher tensile andtear strengths and a slightly higher coefficient of friction.
Thelower compressive modulus of the material results in greater deflection underthe operating preload of the system. The increased deflection greatly reducesthe bondline fatigue of the wheel system by distributing the stress from theoperating loads over a larger area of the bondline. However, the key to the success of the U3000material is its outstanding dynamic properties, specifically its hysteretic propertieswhich, as measured by tan delta, are approximately 35 percent lower than theoriginal NDI-based polyurethane. Hysteretic properties are effectively a measureof the efficiency of a material in handling high loads and strains. The lowerthe tan delta of a material, the less heat is generated within the polyurethaneduring use, and therefore, the higher the load bearing capability of thematerial. Due to the increased deflection of the material resulting from itslower compressive modulus, the improved dynamic properties are required toprevent dynamic failures within the wheel tread. Another enhanced physicalproperty of the U3000 material is its higher tear strength (in excess of 625pliin ASTM D-1938 testing), which effectively resists the severe fatigue crackingthat typically plagues solid elastomeric drive wheels.
Samplewheels with the alternative polyurethane material were manufactured using thecustomer supplied wheels stripped of their NDI tread and recoated. With theU3000 polyurethane chemistry, the retreaded wheels were installed on the systemand have generated significant tangible results, such as:
Extended wheel life: The average wheel life increased to approximately 8 to 12 months of run time,resulting in significant cost savings from reductions in wheel purchases,maintenance costs and downtime issues. Parameters are still being adjusted tooptimize wear rates with the goal of extending lifespan to upwards of 18months.
Reduce system stress/fatigue: The greater fatigue life and higher tearstrength of the material have completely eliminated all fatigue cracking on thedrive wheels and significantly reduced the tread induced vibrations which,coupled with the material's lower compression modulus, have resulted in lessstress and fatigue to the crane structure and drive system thereby yielding alonger life for various components.
Greg Stevens, P.E., is head of research & developmentat Uremet Corporation.
For more information, visit:www.uremet.com
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